Nanocrystalline versus microcrystalline Li2O:B2O3 composites:: Anomalous ionic conductivities and percolation theory

We study ionic transport in nano- and microcrystalline (1 - x)Li2O:xB(2)O(3) composites using standard impedance spectroscopy. In the nanocrystalline samples (average grain size of about 30 nm), the ionic conductivity sigma(dc) increases with increasing content x of B2O3 up to a maximum at x approximate to 0.5. Above x approximate to 0.92, sigma(dc) vanishes. By contrast, in the microcrystalline samples (grain size about 10 mu m), sigma(dc) decreases monotonically with x and vanishes above x approximate to 0.55. We can explain this strikingly different behavior by a percolation model that assumes an enhanced conductivity at the interfaces between insulating and conducting phases in both materials and explicitly takes into account the different grain sizes.